fix autocomplete of usernames (#1019)

app/src/main/java/com/keylesspalace/tusky/adapter/MentionTagAutoCompleteAdapter.java

@@ -76,7 +76,7 @@ public class MentionTagAutoCompleteAdapter extends BaseAdapter
             @Override
             public CharSequence convertResultToString(Object resultValue) {
                 if (resultValue instanceof AccountResult) {
-                    return ((AccountResult) resultValue).account.getUsername();
+                    return formatUsername(((AccountResult) resultValue));
                 } else {
                     return formatHashtag((HashtagResult) resultValue);
                 }
@@ -166,6 +166,10 @@ public class MentionTagAutoCompleteAdapter extends BaseAdapter
         return view;
     }
 
+    private String formatUsername(AccountResult result) {
+        return String.format("@%s", result.account.getUsername());
+    }
+
     private String formatHashtag(HashtagResult result) {
         return String.format("#%s", result.hashtag);
     }